There are a number of known methods for thickening thermoset polyester resin compositions. One of these methods involves the use of inorganic oxides, such as magnesium oxide, to thicken unsaturated polyester resins blended with styrene. Another method involves the coreaction of polyisocyanates with hydroxyl groups contained within the polyester resin or added as a coreactant to form urethane linkages. A third method involves the trimerization of polyisocyanates with the use of a catalyst. A problem with the metal oxide or hydroxide prior art methods is that an inherently stable viscosity plateau is not achieved, and the molding compounds thickened by these techniques have relatively short shelf lives and do not remain soft and pliable for sufficiently long periods of time. A problem with polyisocyanate prior art methods is that the polyisocyanates are sensitive to water and to acid functional groups found in the molecular structures of many resins. These problems are overcome by the present invention which involves the use of an aziridine compound as a thickening agent.